The specific gravity of a material is the ratio of its density to the density of pure water at 4° C. In the wood products industry, the specific gravity (or basic density) of wood is a characteristic that has implications throughout the processing of a cut tree into lumber or other wood products. The basic density of wood is based on oven-dried weight, and is related to the strength, modulus of elasticity (MOE), and pulp yield of the wood, as well as the susceptibility of the wood to uptake additives for treatment and other qualities known to those of skill in the art.
At the beginning of the processing of a tree into a wood product (e.g., lumber), the felled trees are loaded as logs onto trucks. Because all logs are not identical in composition, they are purchased based on a number of factors, including the size, shape, and weight of the logs.
Regarding tree-to-tree variation in wood characteristics, differences are typically found in the moisture content of logs on a single truck depending on the age of the trees and the freshness of the logs. There can be a 5 to 8% difference in moisture content between logs felled in the morning or in the afternoon from the same tree stand. The oven-dry-weight-based moisture content of a log can be from 50% to 200%, a wide range that leads to difficulties when processing the logs (e.g., at a lumber mill), as will be described below. Moisture content and, to a lesser extent, specific gravity can be estimated at the truck level, log level, and lumber level, but the two values are usually inextricably linked by the measurement technique, and typically no correction is made for specific gravity effects on the moisture content or vice versa.
Traditional methods for measuring moisture content include non-contact capacitance measurements and post-processing weight comparisons of pre- and post-oven-dried wood specimens. Techniques for deriving moisture content from non-contact sensors are often influenced by specific gravity, and corrections for specific gravity differences are not typically performed during wood processing.
Growth rate or rings per inch estimated from lumber end faces are used to predict several lumber properties, including specific gravity and moisture content. However, using rings per inch to measure specific gravity suffers from accuracy problems, particularly if the lumber originates from trees of uniform age (e.g., plantation-grown trees) because the end-face rings-per-inch measurement of such trees are typically homogeneous, leading to a lack of variation in estimated characteristics, even though there is likely variation in certain properties (e.g. specific gravity). Without discernable variations between logs, batching of logs based on characteristics, such as specific gravity, is difficult.
During lumber processing, lumber is transported in both linear (high speed) and transverse (low speed) processes. In traditional processing, the rings per inch observed at lumber ends are evaluated during low-speed transverse transport. Within-lumber variations of specific gravity can be very large, so a specific gravity determined based on a rings per inch measurement at the lumber end face may not be representative of the specific gravity of the entire lumber piece.
Other specific gravity (or basic density) measurement techniques include weighing, x-ray, and gamma ray techniques, all of which are complex, expensive, and affected by moisture content. The density measurement in a mill environment is usually based on as-is weight without normalizing moisture content. The effect of moisture content on the specific gravity measurement can be adjusted using a moisture content determined using a moisture meter. However, most moisture meters are accurate only within a 30% range (e.g., 5% to 35%) and report null values for moisture contents outside the range. Thus, when the moisture content of the lumber lies outside this range, such types of moisture meters do not provide moisture content measurements that should be relied upon to adjust specific gravity measurements.
The sorting of wood by specific gravity is desirable during processing of logs into lumber because wood having similar specific gravity should be similarly processed (e.g., dried at a similar rate and time to produce uniformly dried lumber). Variations in specific gravity from board to board during the lumber drying process can cause some boards to become over dried and some to be under dried, which leads to inconsistencies in the properties and shape of the dried lumber and severe drops in value of the lumber.
Additionally, the specific gravity distribution within a particular piece of lumber partially determines the amount of additives that can be absorbed into the wood during a treatment process. Under-treated lumber will be downgraded and over-treated lumber costs more to treat due to the expense of chemicals, handling, and transportation. Thus, knowledge of the distribution of specific gravity of boards, lumber, and other wood products allows for grouping of similar types of material into a batch for treatment, which will yield more uniformly treated products.
The determination of specific gravity independent of moisture content has several other advantages when processing wood products. First, the shrinkage properties of wood vary depending on the specific gravity of the wood (e.g., high specific gravity wood tends to shrink more in the transverse direction than the longitudinal direction, and low specific gravity wood tends to shrink more in the longitudinal direction than the transverse direction). Adjustments to lumber dimensions during processing can be made based on the specific gravity of the wood so as to produce uniformly dimensioned lumber across different specific gravity ranges.
A second benefit of specific gravity determination is that the commonly used capacitance method for determining moisture content can be corrected using specific gravity if such a value is known at the time of capacitance measuring. Most commercially available capacitance-based moisture content sensors includes a correction for specific gravity, yet such a correction is not typically used because the moisture content is measured early in the wood processing and the specific gravity is not then known to any level of accuracy.
A simplified method for measuring the specific gravity of wood independent from the moisture content of the wood would provide several advantages to the wood-processing industry.